About 70% of the commercial gasoline in China comes out from a heavy oil fluid catalytic cracking (FCC) processes, since feedstock of heavy oil contains a great number of sulfur, nitrogen and oxygen heteroatom compounds as well as colloidal asphaltenes, fluid catalytic cracking gasoline not only has a high content of sulfur, but also has a high content of olefin components, in the commercial gasoline, more than 90% of sulfur comes from the fluid catalytic cracking gasoline, which makes sulfur content of China's gasoline much higher than that of foreign gasoline. Thus, how to reduce sulfur content in the fluid catalytic cracking gasoline is a key for reducing sulfur content in the commercial gasoline. Especially, State V gasoline quality criteria which will be implemented nationwide on Jan. 1, 2018 requires that sulfur content in gasoline should be no more than 10 ppm and olefin content should be no more than 24%, exploration and promotion of techniques for deep desulfurization of gasoline has become a pressing demand for the petroleum refining industry.
Desulfurization techniques used by the petroleum refining industry are divided into hydrodesulfurization technique and non-hydrodesulfurization technique, currently hydrodesulfurization is the main approach for desulfurization. For instance, Sinopec Research Institute of Petroleum Processing developed an FCC gasoline selective hydrodesulfurization process (RSDS-I) in 2001, where FCC gasoline is firstly cut into a light fraction and a heavy fraction at a cutting temperature of 90° C., and then the light fraction is subjected to alkali extraction mercaptan removal, and the heavy fraction is subjected to selective hydrodesulfurization using a main catalyst of RSDS-I and a protective agent of RGO-2; and in a second generation of FCC gasoline selective hydrodesulfurization technique (RSDS-II) for improvements on the process above, a cutting point of the light fraction and heavy fraction is decreased to 70° C., and a second generation of hydrogenation catalysts RSDS-21 and RSDS-22 are used in a selective hydrodesulfurization portion of the heavy fraction.
Axens Corporate of French Institute of Petroleum (IFP) developed a Prime-G+ process, where a process flow of full range pre-hydrogenation, the light and heavy gasoline cutting and heavy fraction selective hydrodesulfurization is used, and the cutting temperature is set between 93-149° C. according to a target value of sulfur content, and during the full range pre-hydrogenation process, light sulfide reacts with diolefin in the presence of a catalyst of HR845 to form sulfide with a high boiling point, thus olefin is not saturated; furthermore, two catalysts of HR806 and HR841 are used in the selective hydrodesulfurization of the heavy fraction, thus the operation is more flexible.
Sinopec Fushun Research Institute of Petroleum and Petrochemicals developed an OCT-M process, where FCC gasoline is cut into a light fraction and a heavy fraction at a cutting temperature of 90° C., in which the light fraction is subjected to mercaptan removal and the heavy fraction is subjected to selective hydrodesulfurization using a combined catalyst of FGH-20/FGH-11.
Hai shun de Special Oil Co., Ltd developed an HDDO series diolefin removal catalyst, an HDOS series deep hydrodesulfurization catalyst, an HDMS series mercaptan removal catalyst and a corresponding FCC gasoline selective hydrodesulfurization process (CDOS), where FCC gasoline is firstly subjected to a diolefin removal reaction at a relatively low temperature in a hydrogen condition, then the FCC gasoline is cut into light and heavy components, the heavy fraction is subjected to deep hydrodesulfurization, and the hydrogenated heavy fraction is reconciled with the light fraction to obtain a clean gasoline with less sulfur.
The above techniques have a common problem that the light fraction formed by the cutting has a low yield, and there are fewer components having a content less than 10 ppm, and it is difficult to reduce sulfur content of the light fraction below 10 ppm by means of mercaptan removal only; when gasoline products having sulfur content less than 10 ppm are produced, a majority of light fraction still need to be hydrodesulfurized, thus loss of octane number of full range gasoline is higher (for instance, up to 3.0-4.0). Furthermore, even though the sulfur content can be made less than 10 ppm by means of hydrodesulfurization, there are still the drawbacks that investment and operational costs are high, and a large number of olefin is saturated while sulfide is removed, which not only increases hydrogen consumption, but also reduces octane number of gasoline greatly.
The non-hydrodesulfurization technique is further classified as adsorption desulfurization, oxidation desulfurization, extraction desulfurization, and biological desulfurization techniques, etc., the adsorption desulfurization technique which has been studied widely so far is one of potential methods for deep desulfurization with low energy consumption and almost no loss of octane number since it is carried out at room temperature and atmospheric pressure.
An IRVAD technique jointly developed by Black & Veatch Pritchard Inc. and Alcoa Industrial Chemicals employs multi-stage fluidized bed adsorption, which uses an alumina substrate selective solid adsorbent to process liquid hydrocarbons, during the adsorption, the adsorbent is in countercurrent contact with the liquid hydrocarbons, the used adsorbent countercurrently reacts with a recycled hot gaseous flow (such as hydrogen) to be regenerated. The desulfurization rate of this technique can reach above 90%, however, this adsorbent is of less selectivity, sulfur adsorption capacity thereof is small, and the regeneration process is relatively complicated.
Philips Petroleum Company developed an S-Zorb process where a specific adsorbent is used for desulfurization in a hydrogen condition, the adsorbent takes zinc oxide, silicon dioxide and aluminium oxide as a carrier and loads metal components such as Co, Ni and Cu, etc., which can absorb a sulfur atom in sulfide to maintain it on the adsorbent, whereas the hydrocarbon structure part of the sulfide is released back to the process stream so as to realize a desulfurization process. This process does not generate H2S during the reaction, thereby preventing H2S from reacting with olefin again to generate mercaptan. However, the desulfurization technique places a relatively harsh requirement upon process operation conditions, the temperature of the desulfurization reaction is 343-413° C. and the pressure is 2.5-2.9 MPa.
The adsorption desulfurizer described above cannot be satisfactorily used in the selective hydrodesulfurization of the heavy fraction due to problems such as limited deep desulfurization and small sulfur adsorption capacity, low selectivity, short lifespan, relatively complicated regeneration process and harsh desulfurization conditions. Thus, there is a pressing demand to develop a method for desulfurization of gasoline, of which loss of octane number is less, desulfurization degree is highly deep, and the operation is convenient and flexible.